COMBINATION OF SALTS AND PROTEINS : III. THE COMBINATION OF CuCl₂, MgCl₂, CaCl₂, AlCl₃, LaCl₃, KCl, AgNO₃ AND Na₂SO₄ WITH GELATIN.

1. The combination of Cu⁺⁺, Ca⁺⁺, Mg⁺⁺, Al⁺⁺⁺, La⁺⁺⁺, K⁺, Ag⁺, and Cl^- with gelatin has been determined.

2. The equivalent combining value for copper is about 0.9 millimols per gm. of gelatin and is therefore the same as that of hydrogen. The value for copper with deaminized gelatin is about 0.4 to 0.5, again the same as that of hydrogen. The sum of the hydrogen and copper ions combined in the presence of an excess of either is 0.9 millimols showing that there is an equilibrium between the copper hydrogen and gelatin and that the copper and hydrogen are attached to the same group.

3. The equivalent combining value of La⁺⁺⁺ and Al⁺⁺⁺ is about 0.5 millimols per gm. of gelatin. This value is not significantly different with deaminized gelatin so that it is possible these salts combine only with groups not affected by deaminization.

4. No calcium is combined on the acid side of pH 3. The value rises rapidly from pH 3 to 4.7 and then remains constant.

5. No combination of K, Li, Na, NO₃ or SO₄ could be detected.

6. Cl combines less than the di- and trivalent metals so that the protein is positive in CaCl₂ but negative in KCl.